Storage media are used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape and optical disks. For most media, a head assembly, which includes one or more read transducer heads, reads data from the storage medium. Many forms of media, such as magnetic tape and rewritable optical disks, allow data to be written to the medium as well as read from the medium. A write transducer head or a read/write transducer head writes data to the storage medium.
The data stored on the medium are usually organized into “data tracks,” and the transducer heads write data to and/or read data from the data tracks. A typical storage medium includes several data tracks.
As the number of data tracks on a medium increases, the data storage capacity of the medium increases. The data tracks also usually become narrower, as more data tracks are crowded onto the recording surface of the medium.
For proper data storage and recovery, the transducer head must locate each track where data are to be written or read, and follow the path of the data track accurately along the media surface. A servo controller typically is provided to control the positioning of the head relative to the data tracks.
Earlier servo controllers positioned the heads relative to the expected position of the medium as the medium passed the head. Other servo controllers measured the position of a data track relative to an edge of the recording medium. As more and narrower data tracks were added to media, however, these methods of locating data tracks became less satisfactory and were not able to support the precision needed for head placement.
A more modern approach was to place, during the medium manufacturing process, pre-recorded servo position information at pre-selected sites on the medium. This servo position information was then used by the servo controller to control head motion when seeking between tracks, and to regulate head position on a track during reading and/or writing.
With some forms of storage media such as magnetic tape, the servo information is stored in specialized tracks in the medium, called “servo tracks.” Servo tracks serve as references or landmarks for the servo controller. Servo tracks hold no data except for information that is useful to the servo controller.
The servo tracks are sensed by a servo read head. A servo read head may be a dedicated head that reads only servo tracks and does not read data tracks. Once a particular servo track is located by the servo read head, a data track can be located on the medium according to the data track's displacement from the servo track.
Other forms of storage media, such as magnetic disks, store servo information in one or more specialized sectors. In a magnetic disk, for example, servo information may be recorded in a single sector that extends from the outer circumference of the disk to the inner circumference, or may be recorded in multiple zones or wedges. The servo controller uses the information stored in the servo portion of each sector first to locate the desired track and then to appropriately position the transducer head with respect to the centerline of the desired track.